Method for calibrating a flying height tester utilizing a wedge slider and transparent disc held together

ABSTRACT

A calibration standard for uniformly calibrating flying height testers. The calibration standard includes a wedge slider held in contact with a glass disc by a load bridge, load spring and cover case. The wedge slider has a first rail and a second rail extending along its length and has a raised end to form an optical wedge between the glass disc and the first and second rails. A profilometer is used to map any surface irregularities on the surfaces of the first and second rails facing the disc, allowing the calculation of an accurate expected distance, or flying height, between the surfaces and the disc at each horizontal location along the length of the wedge slider. The expected flying height is compared to a flying height measured through optical interference techniques to calibrate the flying height tester.

This is a division of application Ser. No. 08/184,995, filed Jan. 24,1994, now U.S. Pat. No. 5,410,402.

BACKGROUND OF THE INVENTION

In the disc recording art, it is common to use read/write heads whichreact against the air moved with the rotating disc, causing the heads to"fly" a small distance from the disc surface. In the manufacture of suchread/write heads, it is common to test the hydrodynamic characteristicsof the heads so that the flying height characteristics are known,thereby avoiding the use of heads which fly too high or too low inrelationship to disc surface and also avoiding heads which fly at animproper angle to the disc. Too high a flying height will result inlower areal density, while too low a flying height can cause head/discinterface failure.

Flying height testing is generally accomplished by means of a flyingheight tester, using optical interference techniques. Such a flyingheight tester comprises, for example, a monochromatic light sourcedirecting monochromatic light at a glass disc. The glass disc is rotatedat speeds simulating the rotation of a magnetic disc, and the headassembly being tested is positioned in a holder in flying relation tothe glass disc. Monochromatic light is directed at the disc at apredetermined angle to the surface thereof. Light is reflected from thesurface of the disc closest to the flying head, as well as from thesurface of the flying head itself, and impinges onto a light sensitivesensor.

The flying height between the head and the disc can be determined by theintensity of the light for a monochromatic light source, or by theconstructive or destructive wavelength of the light for a white lightsource. A computer is programmed to receive data from the flying heighttester and calculates the perceived flying height and angle of the head.With the rapid advance of disc drive technology, the flying height ofmany modem disc drives is less than 0.1 microns. Therefore, the accuracyof the flying height tester, and therefore its calibration, is animportant concern.

Calibration of flying height testers has been accomplished through theuse of a standard head whose characteristics are known. However, afterrepeated use, the reflective surface and flying characteristics of thehead are altered by dust, oil and other foreign matter, altering thecalibration of the standard. Calibration of flying height testers hasalso been accomplished through the use of a standard comprising asubstrate having a reflective layer deposited thereon to represent thehead and a transparent layer having a predetermined thickness depositedon the reflective layer. The standard is then placed in the flyingheight tester with the transparent layer spaced from the disc andmonochromatic light is directed at the standard. A disadvantage of sucha standard is that it uses a transparent material rather than airbetween the disc and the reflective layer. In addition, such a standarddoes not provide for the accurate determination of position along thelength of the standard.

SUMMARY OF THE INVENTION

The present invention provides a calibration standard for the accurate,uniform calibration of flying height testers. The calibration standardof the present invention comprises a wedge slider held in contact with atransparent disc through the use of a load bridge, a load spring and acover case. One end of the wedge slider is raised, thereby creating anoptical wedge between the wedge slider and the disc. The cover caseprovides a sealed environment free of dust and other contaminants.

The wedge slider has both a first rail and a second rail, each of whichextends along the length of the wedge slider and has a surface facingthe disc. The first rail has a plurality of cylindrical portions thereinat regularly spaced intervals. Each cylindrical portion has a diameterthat is equal to the diameter of the beam spot from the light source ofthe flying height tester, thus allowing the beam spot to be matchedexactly at the position of any given cylindrical portion. This allowsthe uniform calibration of multiple flying height testers with a singlestandard as well as the calibration of the same tester at differenttimes. Measurement errors are also minimized as the exact location ofeach of the cylindrical portions is known.

The second rail has a width that is greater than the diameter of thebeam spot, thereby allowing for a continuous optical wedge measurementalong its length. The second rail also has a plurality of marks on oneside which can be used to determine the position along the length of thewedge slider at which a measurement is taken.

To calibrate the flying height tester, the distance or flying heightbetween the wedge slider and the disc must be measured at a plurality oflocations along its length and compared with known, or expected, valuesof the flying height at those locations. The flying height is measuredat a plurality of locations through the use of known opticalinterference techniques. The expected value of the flying height at eachposition along the length of the wedge slider is calculated using theknown dimensions of the wedge slider. This calculation is then correctedfor any surface irregularities found during a mapping of the surface ofthe first and second rails.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a portion of a standardflying height tester for measuring flying height characteristics ofread/write heads.

FIG. 2 is a schematic cross-sectional view of a calibration standard ofthe present invention.

FIG. 3 is a schematic view of a portion of a calibration standard of thepresent invention.

FIG. 4 is a schematic cross-sectional view of a portion of a calibrationstandard of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a portion of a flying height tester 10 which is used totest flying height characteristics for magnetic heads. A transparentdisc 11 is rotated about its axis so that a magnetic head 12 "flies"above the surface 14 of the disc 11 due to air flow in the direction ofarrow 13 caused by the rotation of the disc 11. Monochromatic light froma light source (not shown) is directed along a path through the disc 11and is reflected off surface 14 of disc 11 and surface 16 of head 12toward a light sensitive sensor. Specifically, the monochromatic lightimpinges disc 11 at an incident angle θ to surface 18, following path20a to surface 18 of disc 11 and path 20b through the disc 11. The lightis split and partially reflected off surface 14 to follow path 20cthrough the disc 11 and thereupon path 20d to the sensor. The otherportion of the split light follows path 20e to reflect off surface 16 ofhead 12 and follow path 20f to surface 14 of the disc 11, path 20gthrough the disc 11 and path 20h to the sensor. The slight angulardeviations at the interface between air and disc are caused by the Snelleffect. It should be noted that paths 20a and 20d are each oriented atan angle θ from the vertical and that path 20h is not parallel to path20d but is at an angle θ+2α to the vertical, where angle α is the angleof orientation of the flying head 12 with respect to surface 14.

Path 20a represents only one of numerous parallel beams of light used inthe tester. The sensor, therefore, will receive light from the paths 20dand 20h corresponding to each of the numerous paths 20a. As a result,the light received by the sensor will include, at any given location,light from a path 20d from one beam and light from a path 20h fromanother beam. The distance d between the head 12 and the surface 14 ofthe disc 11 can be determined by the intensity of the light impinging onthe sensor for a monochromatic light source, or by the constructive ordestructive wavelength of light for a white light source. The angle αcan be determined by finding the distance d at a plurality of points.

The calibration standard 30 of the present invention is shown in FIG. 2and comprises a glass disc 32, a wedge slider 34, a load bridge 36, aload spring 38 and a cover case 40. The wedge slider 34 contacts thedisc 32 at both a first end 41 and a second end 42 and is held incontact with the disc 32 through the use of the load spring 38, the loadbridge 36 and the cover case 40. The cover case 40 contacts the glassdisc 32 and is held in place through the use of an adhesive. A first end43 of the load spring 38 contacts an inner surface 44 of the cover case40 while a second end 45 of the load spring 38 contacts a top surface 46of the load bridge 36. The load bridge 36 has a first leg 47 and asecond leg 48 which contact the wedge slider 34 at its first end 41 andsecond end 42, respectively, so as to transmit the force from the loadspring 38 to the wedge slider 34 without deforming the wedge slider 34.The load spring 38 is designed to provide sufficient pressure to firmlyhold the wedge slider 34 against the disc 32 without causing deformationof the wedge slider 34 or the disc 32.

A pair of bumps 49 at the second end 42 of the wedge slider 34 create awedge-shaped space between the wedge slider 34 and the glass disc 32.The bumps 49, which can be seen in more detail in FIG. 3, each have agenerally flat surface 50 which contacts with the disc 32. In additionto providing a backing for the load spring 38, the cover case 40provides a sealed environment for the wedge slider 34, thereby reducingthe deterioration of the wedge slider 34 through the build-up of dustand other contaminants. The wedge slider 34 is generally fabricated fromthe same material as that from which the sliders to be measured arefabricated. This eliminates any error that might occur due todifferences in the properties of the materials.

The wedge slider 34 is shown in more detail in FIG. 3, and is shown ashaving both a first rail 52 and a second rail 54, each of which extendsbetween the first end 41 and the second end 42 of the wedge slider 34.The first rail 52 includes a plurality of cylindrical portions 56located at regularly spaced intervals along its length each of which hasa first end 57 which is a part of a generally flat surface 58 of thefirst rail 52. The second rail 54 has a plurality of regularly spacedprotrusions 60 extending from a first side 62 and has a generally flatsurface 64. Both the first rail 52 and the second rail 54 extend thesame distance from a first surface 55 of the wedge slider 34 so that thesurfaces 58 and 64 are in the same plane. In addition, each of the bumps49 extends the same distance from the first surface 55.

The position of the wedge slider 34 on the glass disc 32 is shown indetail in FIG. 4. The horizontal position along the length of the wedgeslider 34 is defined by the variable x which is measured along an x-axisthat begins at the first end 41 of the wedge slider 34 where x=0 andends at the second end 42 of the wedge slider 34 where x=L. The verticaldistance, or flying height, between the glass disc 32 and the surfaces58 and 64 of the first and second rails 52 and 54 is defined by thevariable h. The flying height h varies with x and is zero at x=0 and Hat x=L.

To calibrate a flying height tester using the calibration standard 30 ofthe present invention, a measured flying height h is compared with aknown, or expected, value of h at a plurality of points along thex-axis. To do this, it is first necessary to determine an expectedflying height h between the surfaces 58 and 64 of the first and secondrails 52 and 54 and the glass disc 32 at each point along the x-axis.

If the surface 58 of the first rail 52 and the surface 64 of the secondrail 54 were perfectly flat, an expected value of the flying height h ofthe wedge slider 34 could be easily determined at each point along thex-axis through the use of the equation h=xH/L. However, in even the mostcarefully fabricated wedge slider 34, the surfaces 58 and 64 of thefirst and second rails 52 and 54 will not be perfectly flat. Rather,they will have small, almost imperceptible, surface flaws. To compensatefor these surface flaws and the resulting errors they can cause in thedetermination of the flying height h, the surfaces 58 and 64 of thefirst rail 52 and the second rail 54 are mapped through the use of aprofilometer. Generally, the results of this mapping are stored in acomputer as a correction function and an appropriate correction value iseither added to or subtracted from the flying height h given by theequation h=xH/L at each horizontal position. Through the use of thecorrection values, the exact flying height h of the surface 58 of thefirst rail 52 and the surface 64 of the second rail 54 is known for eachvalue of x. The mapping of the first and second rails 52 and 54, as wellas the measurement of L and H, is generally done prior to thepositioning of the wedge slider 34 in the calibration standard 30.

To measure the flying height h of the wedge slider 34, the calibrationstandard 30 is positioned in the flying height tester adjacent to alight source 72 of the tester. To do this, the disc 11 of the flyingheight tester is removed and the glass disc 32 along with the rest ofthe calibration standard 30 is inserted. Generally, the calibrationstandard 30 is mounted with the glass disc 32 facing upward, thecalibration standard 30 being inverted from the orientation shown inFIG. 2.

Once the calibration standard 30 is in place, a continuous optical wedgemeasurement is made on the second rail 54 from the horizontal positionx=0 to the horizontal position x=L. At each point along the x-axis,monochromatic light is directed at the glass disc 32 from the lightsource 72 along path 76a, as shown in FIG. 4. The light impinges thedisc 32 at an incident angle θ to a first surface 80 of the disc 32 andcontinues through the disc 32 along path 76b to a second surface 82,where it is divided and partially reflected. The reflected portionfollows path 76c through the disc 32 to first surface 80, and thenfollows path 76d to the flying height tester sensor (not shown). Theremaining light follows path 76e to the surface 64 of the second rail 54where it is reflected and directed at the glass disc 32 via path 76f.The light impinges the disc 32 at its second surface 82, follows path76g through the disc 32 and then follows path 76h to the sensor (notshown). The slight angular deviations between paths at the interfacebetween the air and the disc 32 are due to the Snell effect. Because theeffects of the Snell effect are self-canceling, light path 76e isparallel to the path 76a and path 76h is parallel to path 76f. Both theheight H and the incident angle θ have been exaggerated in FIG. 4 forillustrative purposes. Path 76a is actually substantially normal to disc32. The height H is generally between 12 and 13 microinches, while thelength L is generally 100 mils.

The light source 72 is moved along the x-axis so that light is reflectedoff of each point along the surface 64 of the second rail 54. Theadditive and subtractive nature of the reflected light along paths 76dand 76h creates a continuous spectrum containing segments of highintensity light as well as darker segments for the sensor. From thiscontinuous spectrum can be found values for both the maximum intensityI_(max) and the minimum intensity I_(min) of the light on the sensor.This process is known as light intensity calibration. The surface 64 ofthe second rail 54 is fabricated so that it has a width greater than thewidth of the beam spot from the light source 72. Therefore the entirebeam spot will be reflected from the second rail 54, allowing anaccurate measurement of I_(max) and I_(min).

Assuming normal incidence to the disc 32 and neglecting the angle α,which is generally very small, the total intensity I of the lightshining on the sensor is given by the equation: ##EQU1## where I₁represents the intensity of the light impinging on the sensor from thepath 76d of one beam, while I₂ represents the intensity of the lightimpinging on the sensor from the path 76h of a second beam. Thisequation represents the relationship between the flying height h of thewedge slider 34 and the intensity I of the light impinging on thesensor. Both I₁ and I₂ are constants and are defined in terms of I_(max)and I_(min) by the equations: ##EQU2## where I_(max) and I_(min) areconstants whose values are determined through the light intensitycalibration procedure discussed above. The value of φ from the firstequation is also a constant and is determined by the equation: ##EQU3##where n and k represent the index of refraction and coefficient ofextinction, respectively, of the material used to fabricate the wedgeslider 34. Once the values of I₁, I₂ and φ are determined, the firstequation can be used to determine the flying height h from measurementsof the intensity I of the light impinging on the sensor. The aboveequations for I, I_(max) and I_(min) are used for a single reflectionfrom the wedge slider 34. For situations involving multiple reflections,the principle and procedure remains the same, but the equations must beslightly modified to account for the multiple reflections.

Once the constants in the first above equation are determined, theflying height h can be measured. The flying height h is generallymeasured at ten or more locations along the length of the wedge slider34. For each location, the intensity I of the light impinging on thesensor is measured and used to calculate the flying height h. Themeasured flying height h can then be compared to the expected flyingheight to calibrate the flying height tester.

For an accurate measurement of the flying height h, a knowledge of theexact location along the x-axis at which the flying height h is beingmeasured is extremely important. It is for this reason that thecylindrical portions 56 are provided on the first rail 52 of the wedgeslider 34. Each cylindrical portion 56 has a diameter that is equal tothat of the beam spot from the light source 72, allowing the beam spotto be matched exactly at the position of any given cylindrical portion56. The flying height h of the wedge slider 34 can therefore be measuredat a position along the x-axis that is known exactly. Without the use ofthe cylindrical portions 56, the position along the x-axis at which theflying height h is to be measured must be measured from the first end 41of the wedge slider 34 and the introduction of a measurement error islikely. Because the exact location along the x-axis of the cylindricalportions 56 is known and remains constant, a single wedge slider 34 canbe used to calibrate more than one flying height tester with greatconsistency. Also, measurements of the flying height h can be taken morequickly than if the horizontal location had to be measured from thefirst end 41 of the wedge slider 34.

While measuring the flying height along the first rail 52 has theadvantage of the use of the cylindrical portions 56, the flying heightcan also be measured along the second rail 54. The regularly spacedprotrusions 60 of the second rail 54 act as reference marks with knownlocations to help increase the accuracy in determining the locationalong the x-axis at which the flying height h is being measured.

The wedge slider 34 is fabricated through the use of well-knownthin-film deposition techniques. During fabrication, a block of the samematerial as that of the wedge slider 34 can also be fabricated. Thispiece of material will generally have the same optical properties as thewedge slider 34 and is used to determine the index of refraction n andthe coefficient of extinction k of the material used to fabricate thewedge slider 34. These optical properties n and k are then used todetermine the constant φ through the use of the equations given above.With the increasingly small flying heights h of today's magnetic heads,it is increasingly important to accurately consider the effects of theoptical properties of the wedge slider 34 in calibrating the flyingheight tester.

The bumps 49 of the wedge slider 34 can be made sufficiently high toallow calibration of the flying height tester in any of a variety offlying height ranges by providing a wedge having a continuous spacingvariation from zero to a selected H. While current flying heights aregenerally less than one tenth of a micron, it is possible to make awedge slider 34 having a large enough wedge spacing to calibrate flyingheight testers used to test older heads which generally have much higherflying heights.

While calibration of the flying height tester has been described withreference to light from a monochromatic light source, both polychromaticand white light may also be used. For polychromatic light, thecalibration of the flying height tester is performed in exactly the samemanner as that described for monochromatic light. The only difference isthat the sensor can independently detect the intensity of the lightimpinging thereon for each wavelength. For white light, the constructiveor destructive wavelength of light is used to determine the flyingheight h.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method of calibrating a flying height testerthat uses optical interference techniques to determine a flying heightof magnetic heads, the method comprising the steps of:inserting acalibration standard in the flying height tester, the calibrationstandard having a transparent disc and a wedge slider forming agenerally Wedge-shaped space between the wedge slider and the disc;determining an expected distance between the disc and a facing surfaceof the wedge slider at a plurality of locations along a length of thewedge slider; determining a measured distance between the disc and afacing surface of the wedge slider through optical interferencetechniques at each of the plurality of locations along the length of thewedge slider at which an expected distance was determined; and comparingthe expected distance with the measured distance at at least one of theplurality of locations.
 2. The method of claim 1 wherein the wedgeslider has a first end, a second end and a first rail extending betweenthe first and second ends, the first rail having a first surface facingthe disc.
 3. The method of claim 2 wherein the facing surface of thewedge slider is the first surface of the first rail.
 4. The method ofclaim 3 wherein the first rail has a plurality of cylindrical portionstherein, each of which has a first end facing the disc and a diameterequal to a diameter of a beam of light produced by a light source in theflying height tester.
 5. The method of claim 4 wherein the locationsalong the length of the wedge slider at which the measured distance isdetermined are the locations of the cylindrical portions of the firstrail.
 6. The method of claim 5 and the additional step of determining atleast one optical property of a material of the wedge slider for use inproving accuracy of the step of determining a measured distance.
 7. Themethod of claim 3 and the additional step of determining a correctionvalue for each location along the length of the wedge slider, whereinthe correction values represent surface irregularities on the firstsurface of the first rail and are used to improve accuracy of the stepof determining the expected distance between the disc and the firstsurface of the first rail.
 8. The method of claim 7 and the additionalstep of mapping the first surface of the first rail using a profilometerto determine any surface irregularities of the first surface of thefirst rail.
 9. A method of calibrating a flying height tester that usesoptical interference techniques to determine a flying height of magneticheads, the method comprising the steps of:inserting a calibrationstandard in the flying height tester, the calibration standard having atransparent disc and a wedge slider forming a generally wedge-shapedspace between the wedge slider and the disc; determining a measureddistance between the disc and a facing surface of the wedge sliderthrough optical interference techniques at a plurality of locationsalong a length of the wedge slider; and comparing the measured distancewith a predetermined expected distance between the disc and the facingsurface of the wedge slider at each of the plurality of locations alongthe length of the wedge slider at which a measured distance wasdetermined.